Battery/Inverter Bus Bar

Hi,
I'm new to the forum and I'm sure that this has been beat to death but...
My thing is - up until recently I was only using two marine battery's hooked up to provide 12V and 250AH. Controller hooked to opposite ends of the battery bank, the inverter hooked to the other end and a Harbor Freight controller hooked up to run some 12V lights and show the voltage. Easy and simple. Then I came across a source for a used T-1275 battery cheap (now 400AH). Got one and wired it in - now the same guy says he's should have two more for me by next week (700AH).

I have been running either a 750W or a 1200W mod sine inverter for shop equipment, lights, lead melting pot etc.. but, with the increase in amperage from the future batteries should I connect the positive side to a bus (using either 2 or 0 gauge wire) and the same for the negative side, and then use the bus as a connection point for the inverters or just leave well enough alone and keep doing what I have been. Also, if I go to a bus bar - I guess really should put an in-line fuse going to the positive bar. I have been relying on the inverter fuses but w/ a bar setup the amps could blow me across the shed or kill me.

Suggestions/Commets?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Battery/Inverter Bus Bar

    You really need to backup and define the loads you want to power and size the battery bank to support that level of current.

    Then, with the load and battery bank defined, you need to look at your charging situation.

    Note, for solar/battery systems, when you start talking about large batteries and heavy loads, you are are in need of heavier copper cable, fusing, more solar panels, larger/multiple charge controllers and such...

    For example, a 700 AH 12 volt battery bank should (using our typical rules of thumb) would need around 5% to 13% rate of charge (again, these are just starting rules of thumb to give you a rough idea of your power needs):
    • 700 AH * 0.05 = 35 Amp minimum rate of charge
    • 700 AH * 0.10 = 70 Amp "nominal" rate of charge
    • 700 AH * 0.13 = 91 Amp "cost effective maximum" rate of charge
    Solar panel wise:
    • 35 amps * 14.5 volts charging * 1/0.77 panel+charger deratings = 659 watt minimum array
    • 70 amps * 14.5 volts charging * 1/0.77 panel+charger deratings = 1,318 watt "nominal" array
    • 91 amps * 14.5 volts charging * 1/0.77 panel+charger deratings = 1,714 watt "cost effective" maximum array
    Of course, this does not take into the size of the array needed to replace the power you use during the day (limited amount of sun, you may need more solar panels to meet your daily loads).

    This is not a small system... And given that large charge controllers (12/24/48 volt) are around 60-80 amps maximum--You may need 2 charge controllers at 12 volts where a single controller will work at 24 or 48 volts battery bank voltage.

    Also, mixing types/brands/ages of batteries together in parallel (or series, or series/parallel) is not usually a great idea either. Parallel connected batteries may not share current/loads/charging evenly. And series connected batteries may have weak cells in the string (i.e., all 200 AH except for one 100 AH weak cell--the maximum string AH capacity is limited to that one 100 AH capacity cell).

    In any case, just adding batteries to a system causes a whole bunch of other issues (current sharing, size of charge controllers, larger battery bank should have larger array for proper charging current, fusing/breakers, etc.). Usually, I like to recommend that people limit the maximum power from a 12 volt battery bank to ~100 amps or 1,200 watts. A 24 volt battery bank to ~2,400 watts, and use 48 volts for larger loads/charging sources.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Battery/Inverter Bus Bar

    So by adding the T-1275 and possibly the other batteries I have out run my panels and charging capacity.

    I use the setup in my shop, a few tools like the drill press and grinder use 1/2 HP motors and draw about 3.5 - 4 amps when running, most of the smaller hand tools use less. My lead pot only draws 500W max, add another amp for lighting and that is about all I use it for. I started this system to learn so when I do go to a larger setup, I'm not frying batteries and the like.

    I was hoping to accumulate more battery storage since I try not to run the batteries below 12V. My last set of marine batteries lasted right at 4 years using a 90W - 2 battery setup.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Battery/Inverter Bus Bar

    That is impressive--Marine batteries are just a, slightly, heavier duty cousin of the automotive battery. For the most part, I would have to guess you did not take your batteries much below 75% state of charge (or at least did not do it very often).

    Remember that you are talking about 4 amps at 120 VAC... Since Power=Volts*Current; that is ~40 amps at 12 VDC--A much higher current. About 1/2 the starting current for a small car.

    The issues of 4 amps at 120 VAC do get more complex--there is the question of how long you run the motor (drills typically are operated a few minutes or less, and not always near rated load). Also, the "math" behind AC power is actually quite complex. It is possible for a motor to "draw" 4 amps at 120 VAC:
    • 4 IAC * 120 VAC = 460 VA (not watts, Volts*Amps)
    The actual power may be:
    • Power (AC) = Volts * Amps * Power Factor (for motors the Cosine of the lagging current) ~ 4 IAC * 120 VAC * 0.60 = 288 Watts
    So--more or less, the current "lags" the voltage for inductors, motors, and florescent ballasts. This means the AC wiring has to carry more Amps than would have been expected if the Voltage and Current sine waves where exactly in phase (lined up).

    Anyway, the amount of current drawn at 120 VAC is approximately 1/10th of the current drawn at 12 VDC when using an AC inverter (12 VDC to 120 VAC inverter).

    The approximate amount of power you could "reliably" pull from generic deep cycle flooded cell batteries is (very roughly) guessing you started with 200 AH of 12 volt batteries is:
    • 12 volts * 200 AH * 1/8 (8 hour discharge) = 300 watts continuous recommended power (upwards of 4 hours)
    • 12 volts * 200 AH * 1/2.5 (maximum surge current from battery) = 960 watts maximum for short starting surges
    Any way--that is how I would guess your system should have been working assuming the batteries were well charged between uses.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Battery/Inverter Bus Bar

    Yea, I usually stopped drawing power when my batteries showed 12.5V and only a hand full of times did I drag them down to around 12 or 12.2V. The batteries always stayed charged and I checked the water at least once a month.

    According to my Kill-A-Watt™ meter, the drill press or grinder at start up will pull around 6.8A then quickly drop to about 2.3A unloaded and rise to about 3.5A loaded. These are short use items and I watch battery voltage closely when I'm using these. Even though I have seen a 5000W 12V inverter, I had already decided that the 1200W inverter was the largest I was going to use. If I need to run my compressor, table saw or planer it gets plugged into the house or a generator.

    The power consumption #'s are dead-on. Which is why I was hoping to add the batteries to put a larger gas tank on my power reserves. That way, if I ever needed to run the fridge or freezer (if utility power dropped off for longer than 12 hours) or spend more time in my shop - I could. Unfortunately, I didn't realize that w/ with more batteries I would need more charging capacity also. It makes sense, but I had never seen it written anywhere before. I hate to see the T-1275's go unused, but when I factor in the costs of more PV panels and everything else that goes along with it. Even at free - the used batteries won't be cost effective.

    Thank you for the advice.
  • System2
    System2 Posts: 6,290 admin
    Re: Battery/Inverter Bus Bar

    So I never did my orginal question answered. If and when I get the proper amount of PV panels and storage capacity, is using bus bars a good idea to attach inverters?
    I tend to use my 750W inverter most of the time just because it is a little more efficient, the 1200W gets used when I need the larger shop equipment. Only one gets used at a time. The extra lugs can be used to check voltage or for other stuff.

    The bus bars I have found are rated for 350A @ 12VDC when 3/8" lugs are used. I would have one for the positive side and another for the negative side. The largest inverter I am going to use is 1200W. Attachment not found.

    Thanks Drew
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Battery/Inverter Bus Bar

    Sorry--Bus bars are just "handy" ways of connecting/routing current to loads and charging sources. If you can combine a bus bar with fusing/breakers, then it usually makes for a cleaner installation (but probably more expensive).

    wind-sun_2197_35179121

    From our host's Midnite Solar Breaker/Bus bar page.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: Battery/Inverter Bus Bar

    Hi azdrew,

    To me, the pic of the breaker in your last post appears very wimpy. Looks to me, like it woud be a fuse @ 350 Amps.

    Midnite makes a ton of very useful interconnection devices. It is all rated by safety agencies ETL, and the like.

    Our host carries a lot of these items, and can usually order some special item, and have it to you in a week or two it not in stock.

    BB Bill, that MN gutter (I think) sure looks useful. I probably should have used something like that for the current project. Thanks, Vic
    (BTW, just recovered an Auto-Saved post .. almost all of it was there, COOL!!)
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.